Coriolis force sensors, such as gyroscopes for sensing rotational rate, are known. Generally, such sensors include a proof mass coupled to a support by flexures and vibrated in an oscillatory manner in-plane at a predetermined frequency. More particularly, a drive electrode electrostatically couples a drive, or excitation, signal to the proof mass to impart such vibration. The vibrating proof mass is responsive to an inertial input, such as a rotational rate, for deflecting out of the plane of vibration as a result of Coriolis forces induced by the inertial input.
An output sense electrode permits sensing of the out-of-plane deflection of the proof mass for further processing to provide a sensor output signal indicative of the inertial input. Generally, an additional sense electrode is provided for sensing the in-plane displacement of the proof mass caused by the vibration. The in-plane sense signal is coupled to a feedback gain control circuit for controlling the amplitude of the drive signal.
As is known in the art of Coriolis force sensors, in order to achieve an acceptable response from the sensor, the in-plane proof mass vibration has a frequency at, or close to, the resonant frequency of the proof mass. To this end, the drive signal has a frequency equal to the resonant frequency of the proof mass. However, parasitic capacitances between the drive electrode and the sense electrodes can cause significant errors. That is, when the drive signal capacitively couples into the in-plane sense electrode, the accuracy of amplitude control by the feedback circuit is degraded resulting in less than optimum sensor performance. Moreover, when the drive signal is capacitively coupled to the out-of-plane sense electrode, the resulting sensor output signal will be contaminated and will not be an accurate indication of the Coriolis forces induced by the inertial input.
Unfortunately, the motion of the proof mass, or motor, established in the absence of rotation is not completely in-plane. A small but not insignificant out-of-plane motion component, due to mechanical rotation of the suspension beam's principal area moments of inertia and other misalignments, is in time quadrature with the Coriolis motion. This undesired component is typically referred to as "quadrature" and degrades system performance by limiting AC gain in front of a baseband modulator, thus deteriorating the overall DC performance of the system. This is extremely undesirable since the out-of-plane motion is often very small for angular rates of typical interest. Thus, a low sensor gain puts a severe burden on detection electronics in terms of noise and drift. Moreover, the quadrature term of the sensor output signal is multiplied by any phase error introduced by the electronics used to process the sensor output signal, thereby causing the quadrature term to couple into the output.
It has been proposed to minimize unwanted quadrature signals by providing highly accurate phase response in the compensating circuitry. Specifically, a demodulation reference would have to be exactly in-phase with the rate-dependent, in-phase signal. Even if this approach is assumed practicable, it would necessitate the use of undesirably complex and expensive additional circuitry. Thus, a simple means for eliminating the effect of quadrature is desired.
Various techniques are utilized generally in an effort to reduce closed-loop phase error, or drift, in servo circuits, such as amplifier circuits utilizing an operational amplifier. One such technique includes the addition of one or more zeros (i.e., a lead filter) in cascade with the open-loop gain of the operational amplifier in order to flatten the open-loop gain over a portion of the frequency band, generally resulting in only moderate closedloop error reduction and also compromising stability. Another technique for reducing gain and phase errors is to increase the gain-bandwidth product associated with the operational amplifier. However, use of this technique is limited by the gain-bandwidth product of commercially available operational amplifiers as well as by the acceptable increased power dissipation associated with higher performance operational amplifiers.
In a typical micromechanical in-plane gyroscope, a servo system is employed to establish sinusoidal in-plane proof-mass motion of fixed amplitude. Application of an inertial rate about an axis in-plane and orthogonal to the direction of in-plane motion of the proof-masses generates an out-of-plane force also known as a Coriolis force. This out-of-plane force is proportional to the in-plane motor velocity and the inertial rate, and excites out-of-plane motion at the same frequency as the in-plane motion. The amplitude of the Coriolis-induced out-of-plane motion is thus proportional to the input inertial rate. Accordingly, in order to achieve an indication of inertial rate, it is desired to sense that component of the out-of-plane motion which is in-phase with the motor velocity.